In order to overcome space limitations in control points or rooms which might otherwise have a high equipment to space ratio such as the cockpits of commercial and heavy general aviation aircraft, electronic instruments normally have control units and readouts located at the control point, for example, on the cockpit instrument panel, and the body of the instrument located in remote radio racks. Not only does this arrangement overcome the aforementioned control point space limitations but it also facilitates maintenance of the equipment by making the body of the equipment easily accessible at the remote location.
The control of remote equipment from a centralized control point presents some special requirements, especially in aircraft applications. Specifically:
1. The control system must use a minimum of interconnecting cable in order to reduce the weight and the cost of the interconnection.
2. The reliability and integrity of the control system must be high.
3. The time delay introduced by the control system must be short.
4. The control system must be failsafe.
The first three of the above requirements are readily satisfied by serial digital remote control systems. Types of these digital remote control systems are well known in the prior art. The last requirement, that the control system be failsafe can be, and has been, in the prior art, met by any one of the following three means and methods.
According to the first of these methods, an audible identification signal (voice or Morse code) is transmitted, by a ground based facility in the case of aircraft applications. When the system operator hears the correct identification he is assured that the remote unit is actually tuned to the desired channel or mode of operation and that his control system is operating properly. If the identification signal is lost or otherwise no longer heard by the system operator, he knows that a fault has occurred and the instrument is no longer reliably operable. This method is used in VORTAC and ILS systems. Unfortunately, this method increases the operator workload, especially when a Morse code identification signal is used.
According to the second method, the remote unit generates a code signal in response to a control signal received from the control unit. The code signal is transmitted via the control system back to the control unit. The code signal energizes a special display at the control point thereby visually identifying the control signal and verifying for the operator that the remote unit and the control system are operating properly. The cost of implementing this method is relatively high and the reliability and integrity of the control point display introduce other problems.